From macroplastics to microplastics: Role of water in the fragmentation of polyethylene
Introduction
After 60 years of industrial life, plastics have become omnipresent in our lives but their end of use raises concern. It was approximated that in the last 50 years, 12,000 Mt of plastics escaped from the waste management cycle and entered in landfills or directly in the environment (Geyer et al., 2017). Once in the aquatic environment, macro debris of plastic undergo mechanical (erosion, abrasion), chemical (photo-oxidation, hydrolysis) and biological (degradation by microorganisms) modifications (Andrady 2017). All these actions lead to the weathering and the fragmentation of plastic macro debris in smaller and more abundant pieces called microplastics when their size is under 5 mm (Arthur et al., 2009). Theoretically, the final degradation of a polymer would be reached when it is mineralized. The amount of time for a complete degradation of inert plastic polymers such as PE or PP in the marine environment is roughly estimated to several hundreds of years and this degradation is probably the results of several complex processes with various kinetics (Barnes et al., 2009; Gewert et al., 2015). It is particularly difficult to monitor all these processes in the real environment and laboratory studies are still necessary to obtain more accurate data and to identify the pathways leading to an eventual bio-assimilation of plastic debris in the aquatic environment. Among all processes, the abiotic degradation of polymers in the environment leading to their fragmentation is of particular interest. Indeed, a lot of studies aimed to study the loss of mechanical properties in polymer during their ageing but experimental studies on their fragmentation at air or in water are scarce (Jahnke et al., 2017; Kalogerakis et al., 2017) and the ultimate size distribution of polymer fragments that can be generated during environmental fragmentation is not really known raising the question of the possible presence of great quantities of nanoplastics in the future (Koelmans et al., 2017).
The parameters that can modify the fragmentation pathways (polymer nature or structure, environmental conditions) are not totally identified and it is important to verify if some of these parameters can impact the ultimate size of generated fragments. However, monitoring fragmentation in real environment is quite impossible due to the potential loss of generated fragments. In this study, the long-term artificial weathering of polyethylene films was studied in laboratory in order to monitor the first stages of the fragmentation under two different environments: in air or in Milli-Q water. Since this work is a first study to provide understanding of the fragmentation pathways and kinetics, models as simple as possible were selected in order to identify to what extent each parameter (water vs air) impact separately the processes at stake. Polyethylene was chosen as it is the polymer the most commonly found in the environment (Phuong et al., 2016). Fragments are supposed to be generated when cracking lines converge. The key to predict the fragment number and size is then to study the appearance and behavior of cracks in the material. Crack propagation inside a material follows two principal modes differing by their speed of propagation, either named rapid crack propagation (RCP) or slow crack growth (SCG) (Alkhadra et al., 2017; Šindelář et al., 2005). SGC mechanism has been widely studied in polymers, these fractures are characterized by the stable growth of a crack with little deformation in the plastic material. RCP has been mainly studied in mineral material but it has been also observed for HDPE pipes in contact with pressurized gas or chlorine solution (Choi et al., 2009; Frank et al., 2009). The fragmentation mechanism is not completely elucidated for semi-cristalline polymers, especially in water, but observation seems to indicate that cracks are initiated on an impact event and can travel long distances rather quick. The aim of this work is to provide information on the link between oxidation, cracks formation and size of fragments in polyethylene films under weathering and to investigate the role of water in these processes.
Section snippets
LDPE films
Low density Polyethylene (LDPE) films (2 × 2cm) with the same thickness (i.e. 24(3) μm as measured by micrometer) and the same roughness (i.e. 25(5) nm as measured by AFM on 30 × 30 μm2 image) were selected from a film obtained by a blown-extrusion process. To avoid external mechanical stress which can promote fragmentation sample manipulation was as limited as possible. The films were cut to make a distinction between the top face (toward the lamp) and the bottom face (toward the beaker). The
Fragmentation
As shown by Fig. 1, between crossed polarizer, lines parallel to the film extrusion direction can be observed on the as prepared (pristine) LDPE film. These lines originate from the induced stress brought by the process of film formation (passage through the extrusion die and fast coldening) (Zhang et al., 2004). After 25 weeks of accelerated weathering with no manipulation, the replicates in water have fragmented in anisotropic bended fragments with the longer length in a normal direction
Conclusion
This study confirms that the LDPE films weathering, either in water or at air, leads to the introduction of oxygenated groups, chain scission and rearrangement on the uppermost surface of the films. As observed for the air weathered films, above a threshold value the increase of surface hardening prevents the initiation of cracks because voids formation is limited. Since no cracks can be created and propagate, the induced stress can only be released through the wrapping of the film along the
Declarations of interest
None.
Acknowledgements
This work was funded by the French National Research Agency ANR through NANOPLASTICS (ANR-15-CE34-0006-02) and BASEMAN (ANR-15-JOCE-0001-01) projects. We also would like to thank Nadine Auriault and Roxane Noblat from CTTM (Centre de Tranfert et Technologie du Mans) for their help with extrusion process and Cécile Brault for her help in figure drawing.
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